Such a water jet propulsion system for vessels typically has an arrangement in which a single ejection nozzle is fixed to a transversely central part at a stern of a vessel, and water jets to be ejected from the nozzle have an ejection quantity changed, as a rotation speed of an impeller is changed, and an ejection velocity controlled, as a bore diameter of the nozzle is controlled.
The changed ejection quantity of water jets shifts propulsive power of the vessel, and the controlled ejection quantity provides a controlled travel speed.
Accordingly, the ejection quantity and the ejection velocity of water jets are adequately varied in accordance with a weight of loads on the vessel, to thereby enable a saved fuel consumption at a drive for the impeller, allowing an economical travel.
However, the load weight of a vessel has an inseparable relationship to a drafting state of the vessel, such that a lighter load weight provides a shallower draft level, and a heavier load weight provides a deeper draft level.
If the draft is deep, a hull of the vessel has an increased fluid resistance, and besides, water jets hit waves, having a fraction of propulsive power killed, whereas a compensation therefor would cause an inflexible controllability of water jet ejection quantity and ejection velocity.
As a result, the economical travel has conventionally been allowed within a relatively narrow range of load weight.
To this point, there has been proposed an arrangement, which had a pair of ejection nozzles arranged side by side.
This arrangement allowed a sufficient ejection quantity to be secured, with a postponed problem of water jets hitting waves when with a heavy load.
There has been proposed another arrangement, which had a pair of ejection nozzles vertically arranged for a concurrent use, with a wave-hitting problem still left on a lower nozzle.